System for controlling ignition energy of an internal combustion engine

ABSTRACT

A system for controlling ignition energy of an internal combustion engine includes an ignition coil having a primary coil and a secondary coil coupled to first and second electrodes of an ignition plug defining a spark gap therebetween, wherein the primary coil is operable to induce a spark voltage across the secondary coil and across the spark gap. The secondary coil is responsive to the spark voltage to produce a discharge current across the spark gap of the ignition plug. In accordance with one aspect of the invention, means are provided for controllably shunting the primary coil to thereby control the duration of discharge current across the spark gap of the ignition plug. In accordance with another aspect of the present invention, means are further provided for drawing discharge current away from the ignition plug to thereby limit the magnitude of the discharge current within some time period after the transfer of energy from the primary coil to the secondary coil begins. The principles of the present invention are accordingly applicable to control the duration and/or magnitude of ignition plug discharge current to thereby minimize electrode erosion while maximizing ignitability in either a conventional ignition plug-based ignition system or an arc propelling ignition plug-based ignition system.

CROSS-REFERENCE TO RELATED U.S. APPLICATION

The present invention is a continuation-in-part of co-pending U.S.patent application Ser. No. 09/063,142, filed Apr. 20, 1998 and entitledCONTROLLED ENERGY IGNITION SYSTEM FOR AN INTERNAL COMBUSTION ENGINE.

FIELD OF THE INVENTION

The present invention relates generally to ignition systems for internalcombustion engines, and more specifically to controlling spark energy insuch systems.

BACKGROUND OF THE INVENTION

In conventional inductive ignition systems for internal combustionengines, spark plug discharge current is typically characterized by aninitial high current peak followed by a subsequent current decay. Anexample of such a conventional discharge current waveform 150 isillustrated in FIG. 8. One known problem associated with the operationof conventional spark plugs is electrode erosion caused by excessivedischarge currents and/or excessive discharge current duration. Ineither or both cases, electrode erosion leads to poor or at leastreduced engine operational efficiency and, eventually, spark plugfailure.

Another class of ignition systems include specially configured sparkplugs which are operable to propel the arc away therefrom to facilitatecombustion of lean air-fuel mixtures and reduce electrode erosion rate.One example of such a spark plug includes a magnet positioned about theelectrodes, wherein the magnetic field is operable to propel the arcoutwardly from the plug. One embodiment of such a spark plug isdescribed in U.S. Pat. Nos. 5,555,862 and 5,619,959 to Tozzi, which areassigned to the assignee of the present invention, and the disclosuresof which are incorporated herein by reference. With such spark plugs ofthis nature, two key goals are to maximize the ability to ignite fuel atlean air-fuel mixtures while also maximizing electrode life.Unfortunately, the conventional discharge current waveform 150illustrated in FIG. 8 is not optimized to further either of these goals.Excessive discharge current too early in the ignition event results inexcessive electrode erosion while inadequate discharge current near theend of the ignition event results in poor combustion.

What is therefore needed in either a conventional or an arc-propellingspark plug based ignition system is a technique for controlling sparkplug discharge current magnitude and/or duration to thereby maximizeelectrode life, and in an arc-propelling spark plug based system atechnique for further controlling spark plug discharge current near theend of an ignition event to thereby maximize the ability to ignite fueland lean air-fuel mixtures.

SUMMARY OF THE INVENTION

The foregoing shortcomings of the prior art are addressed by the presentinvention. In accordance with one aspect of the present invention asystem for controlling ignition energy of an internal combustion enginecomprises an ignition coil having a primary coil coupled to a secondarycoil, the primary coil responsive to a control voltage to induce a sparkvoltage across the secondary coil, means responsive to a shunting signalfor electrically shorting the primary coil, and a control computerproducing the shunting signal after the spark voltage is induced acrossthe secondary coil, wherein the primary coil is thereafter operable toabsorb the spark voltage and accordingly reduce a duration of the sparkvoltage induced across the secondary coil.

One object of the present invention is to provide an improved ignitionsystem for an internal combustion engine.

Another object of the present invention is to provide such an ignitionsystem operable to control discharge current to thereby minimizeelectrode erosion.

These and other objects of the present invention will become moreapparent from the following description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram of one prior art spark plug for usewith the present invention.

FIG. 2 is a cross-sectional diagram of the spark plug of FIG. 1 viewedfrom a plane 90 degrees rotated from that of FIG. 1.

FIG. 3 is a magnified view of the electrodes of the spark plug of FIG.1.

FIG. 4 is a magnified view of the electrodes shown in FIG. 3 depictingthe flow of current therebetween as the arc is propelled toward theelectrode ends.

FIG. 5 is a plot of discharge current vs. gas density illustrating apreferred range of discharge current operation to prevent electrodedamage while maintaining consistent arc propelling.

FIG. 6 is a plot of discharge current density vs. discharge currentduration illustrating current density value required for consistent arcpropelling.

FIG. 7 is a diagrammatic illustration of one embodiment of thecontrolled energy ignition system of the present invention.

FIG. 8 is a plot of spark plug discharge current over time illustratingsome of the spark energy control techniques of the present invention.

FIG. 9 is a flowchart illustrating one preferred embodiment of asoftware algorithm for controlling the discharge current to a desiredcurrent range following gap ionization.

FIG. 10 is a plot of resistance vs. cylinder pressure illustrating onepreferred technique for mapping current engine load to a desiredresistor value for adjusting the variable resistor shown in FIG. 7.

FIG. 11 is a diagrammatic illustration of an alternate embodiment of thecontrolled energy ignition system of the present invention.

FIG. 12 is a plot of spark plug discharge current vs. time for thesystem shown in FIG. 11 illustrating some of the spark energy controltechniques of the present invention.

FIG. 13 is a diagrammatic illustration of another alternate embodimentof the controlled energy ignition system of the present invention.

FIG. 14 is a plot of primary coil current vs. time for the system shownin FIG. 13.

FIG. 15 is a plot of secondary coil voltage vs. time for the systemshown in FIG. 13 illustrating some of the spark energy controltechniques of the present invention.

FIG. 16 is a plot of secondary coil current vs. time for the systemshown in FIG. 13 illustrating some of the spark plug energy controltechniques of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the preferred embodimentsillustrated in the drawings and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of the invention is thereby intended, such alterations andfurther modifications in the illustrated embodiments, and such furtherapplications of the principles of the invention as illustrated thereinbeing contemplated as would normally occur to one skilled in the art towhich the invention relates.

Referring now to FIGS. 1-4, an example of one prior art arc-propellingspark plug 50 for use with the spark discharge current controltechniques of the present invention is illustrated. In FIG. 1, sparkplug 50 includes a housing 54, typically formed of a metallic material,having a threaded portion 52. Threaded portion 52 enables mounting ofspark plug 50 within a mating threaded hole in a cylinder block of aninternal combustion engine (not shown). Surface 56 of housing 54 mateswith a surface of the cylinder block or cylinder head to form anairtight seal with the combustion chamber formed within the cylinderblock. Terminal electrode 58 is positioned within a bore 62 of aninsulator 60, typically ceramic or similar material, and insulator 60 isfitted into housing 54. A distal end of housing 54 and insulator 64forms a cavity 64 having a first electrode 66 and a second electrode 68formed therein. Electrode 66 is attached to housing 54 in a known mannerand electrode 68 is preferably electrically connected to terminalelectrode 58 via electrode extension 63 and spring 70. In any case,electrodes 66 and 68 form a diverging gap 65 therebetween.

Magnets 72 and 74 (FIG. 2) are positioned within insulator 60 andgenerally surround the cavity 64. Magnets 72 and 74 produce a magneticfield within cavity 64, and hence within spark gap 65, which is operableto urge an arc established between electrodes 66 and 68 within gap 65outwardly toward the end of the spark plug 50 as will be described ingreater detail hereinafter.

Insulator 60 is preferably made from silicon nitride. Magnets 72 and 74are preferably made from samarium cobalt, and housing 54 is made frommaterials typically used in spark plug construction, such as steel orthe like. Electrode 58 is preferably made from nickel and electrodes 66and 68 are preferably made from precious metal materials resistant toarc erosion well known in the art of spark plug construction.

Insulator 60 is not a perfect thermal insulator and a heat sink sleeve71 is preferably provided between magnets 72 and 74 and an inner surface53 of housing 54 to draw heat, generated in the combustion process, awayfrom magnets 72 and 74 toward housing 54. Preferably, sleeve 71 isformed of a material having high thermal conductivity such as copper orthe like.

Referring now to FIG. 3, an enlarged view of electrodes 66 and 68 areshown. The spark gap formed between electrodes 66 and 68 has a narrowgap 76 that diverges to a larger spark gap 78 due to the configurationof electrode 66. Referring to FIG. 4, an enlarged view of electrodes 66and 68 are shown. Various arcs 36a-36c are shown to depict the relativeposition of an arc created and established between electrodes 66 and 68in accordance with various duration levels of ignition signals deliveredto terminal 58 of spark plug 50. In particular, the arc 36a isestablished when a breakdown of the molecules occurs between surfaces66a and 68a of electrodes 66 and 68 respectively, thereby generating aplasma area wherein current flow can be established. The plasma containsions which enable or provide a conduit for current flow. Breakdown ofthe air gap 76 between surfaces 66a and 68a is accordingly oftenreferred to as gap ionization. Once gap ionization occurs, current flowis established in the plasma area created by the ionization event, andarc 36a is accordingly established. When the resistance of the air gap76 is broken down resulting from the ionization event, the voltagerequired to sustain arc 36a typically falls off from the voltagerequired to establish the arc.

Arc 36a may be urged toward the position between surface 66b ofelectrode 66 and surface 68a of electrode 68, designated by the arc 36b,by increasing the level and/or duration of the current I flowing intoelectrode 66. Likewise, the arc may be urged toward the position betweenthe surface 66c of electrode 66 and surface 68b of electrode 68,designated by the arc 36c, by further increasing the level and/orduration of the current I flowing into electrode 66. In either case,inclusion of magnets 72 and 74 significantly reduces the amount ofcurrent required to suitably position the arc between electrodes 66 and68. The force vector, depicted in FIG. 4 as F, is a graphicalrepresentation of the Lorentz force vector acting on arc 36a-c inaccordance with the formula ixB. The diverging gap defined by electrodes66 and 68 provides a means for establishing a variable length arc in aspark plug device, which is particularly advantageous with high pressurealternate fuel engines. An example of one such spark plug 50 isdescribed in U.S. Pat. Nos. 5,555,862 and 5,619,959 to Tozzi, which areassigned to the assignee of the present invention, and the disclosuresof which are incorporated herein by reference.

Alternate fuel engines, particularly propane or natural gas engines,typically operate with lean air-fuel mixtures and cylinder pressures atcombustion that may vary widely with engine load. Generally, cylinderpressure increases with engine load, and the diameter of arc 36a-caccordingly decreases. Thus, whereas the diameter of the arc at lightengine load may result in acceptable surface temperatures of electrodes66 and 68, the diameter of the arc decreases with an increase in engineload so that a correspondingly concentrated arc at high engine load mayresult in surface temperatures of electrodes 66 and 68 that exceed themelting point thereof. In accordance with the present invention, thecurrent flowing between electrodes 66 and 68 is accordingly controlledto provide for a current density J that is less than a maximum currentdensity above which electrode surface temperatures may exceed a meltingpoint thereof under all engine load conditions. The current flowingbetween electrodes 66 and 68 must also be controlled to provide for acurrent density that is greater than a minimum current density belowwhich inconsistent propelling of the arc 36a-c may occur. These twocriteria are illustrated graphically in FIGS. 5 and 6. FIG. 5 showsdischarge current, i of FIG. 4, plotted against gas density which isproportional to cylinder pressure. As illustrated in FIG. 5, waveform 80marks the maximum discharge current boundary above which electrodesurface temperatures may exceed a melting point thereof. Waveform 82marks the minimum discharge current boundary below which inconsistentpropelling of the arc 36a-c may occur. Between waveforms 80 and 82, anacceptable discharge current region is defined for the purposes of thepresent invention. FIG. 6 shows discharge current density plottedagainst discharge current duration. As evident from FIG. 6, thedischarge current density 84 below which inconsistent arc propellingoccurs is a decreasing function of time.

Within the acceptable discharge current region defined between waveforms80 and 82 of FIG. 5, the present invention is concerned with minimizingerosion (due to excessive spark current or power discharge) of surfaces66a and 66b of electrode 66, and of surface 68a of electrode 68 whilemaximizing the ability to ignite fuel at lean air-fuel mixtures.Surfaces 66c and 68b of electrodes 66 and 68 respectively generally donot contribute to the dimensions of the spark gap 76 and 78 (FIGS. 3 and4), and concern over erosion of the surfaces thereof is accordinglylessened. In accordance with the present invention, the dischargecurrent (i of FIG. 4) is preferably controlled to an optimum low currentafter gap ionization occurs, wherein the low current is just above acurrent level required for consistent arc propulsion. When the arc hastraveled a specified distance along the diverging gap 65, the dischargecurrent is gradually increased to an optimum current level at whichignition of the air-fuel mixture may occur. One preferred embodiment ofa system 100 for accomplishing these objectives is illustrated in FIG.7.

Referring now to FIG. 7, a controlled energy ignition system 100includes an ignition coil having a primary coil 102 magnetically coupledto a secondary coil 104 as is known in the art. One end of the primarycoil 102 receives a control signal for activating ignition system 100,and this control signal is provided to an input IN2 of a controlcomputer 112 via signal path 116. Preferably, control computer 112 ismicroprocessor controlled and includes digital signal processingcapabilities as well as a memory portion 146. One end 104a of secondarycoil 104 is connected to one end of spark plug 50 and to one end of avariable resistor 118, and an opposite end 104b of secondary coil 104 isconnected to ground potential, to an opposite end of spark plug 50 andto an opposite end of variable resistor 118. Output OUT1 of controlcomputer 112 is connected to variable resistor 118 via signal path 120for controlling the resistance thereof.

Variable resistor 118 is illustrated in FIG. 7 as a potentiometer havinga wiper connected to one end thereof wherein control computer 112 isoperable to control the position of the wiper via OUT1. It is to beunderstood that the structure of variable resistor 118 shown in FIG. 7represents one embodiment thereof, and the present inventioncontemplates utilizing any known variable resistor structurecontrollable by control computer 112 to thereby adjust the valuethereof. Examples of known resistor adjustment structures and techniquesinclude, but are not limited to, zener diode controlled resistorstructures, so-called R/2R ladder structures, and the like.

End 104a of secondary coil 104 is also connected to, or includesintegral therewith, a voltage sensor 110 that is connected to input IN1of control computer 112 via signal path 114. Voltage sensor 110 ispreferably a known sensor such as that described in co-pending U.S.patent application Ser. No. 08/988,787 entitled APPARATUS AND METHOD FORDIAGNOSING AND CONTROLLING AN IGNITION SYSTEM OF AN INTERNAL COMBUSIONENGINE, filed by Luigi Tozzi and assigned to the assignee of the presentinvention, the disclosure of which is incorporated herein by reference.It is to be understood, however, that for purposes of the presentinvention, voltage sensor 110 may be any known sensor operable todetermine a breakdown voltage, V_(BD), corresponding to the voltagerequired to ionize gap 65 of spark plug 50 as described hereinabove, andprovide a corresponding signal to input IN1 of control computer 112.

The secondary coil 104 preferably includes a number of taps each coupledto a capacitor, wherein charging and discharging of the capacitors iscontrolled by control computer 112. Although four such taps andassociated computer controlled capacitors are shown in FIG. 7, it is tobe understood that system 100 may include any number of taps/capacitors,the purpose of which will be fully described hereinafter. In theembodiment illustrated in FIG. 7, a first tap to secondary coil 104 isconnected to an anode of a diode 122, the cathode of which is connectedto one end of a switch 124 and to one end of a capacitor C1. Theopposite ends of switch 124 and capacitor C1 are connected to end 104bof coil 104. Output OUT2 of control computer 112 is connected to aswitch control input of switch 124 via signal path 126 such that controlcomputer 112 is operable to control the opening and closing of switch124 via OUT2. A second tap to secondary coil 104 is connected to ananode of a diode 128, the cathode of which is connected to one end of aswitch 130 and to one end of a capacitor C2. The opposite ends of switch130 and capacitor C2 are connected to end 104b of coil 104. Output OUT3of control computer 112 is connected to a switch control input of switch130 via signal path 132 such that control computer 112 is operable tocontrol the opening and closing of switch 130 via OUT3. A third tap tosecondary coil 104 is connected to an anode of a diode 134, the cathodeof which is connected to one end of a switch 136 and to one end of acapacitor C3. The opposite ends of switch 136 and capacitor C3 areconnected to end 104b of coil 104. Output OUT4 of control computer 112is connected to a switch control input of switch 136 via signal path 138such that control computer 112 is operable to control the opening andclosing of switch 136 via OUT4. A fourth tap to secondary coil 104 isconnected to an anode of a diode 140, the cathode of which is connectedto one end of a switch 142 and to one end of a capacitor C4. Theopposite ends of switch 142 and capacitor C4 are connected to end 104bof coil 104. Output OUT5 of control computer 112 is connected to aswitch control input of switch 142 via signal path 144 such that controlcomputer 112 is operable to control the opening and closing of switch142 via OUT5. Switches 124, 130, 136 and 140 may be any knownelectrically controllable switches, and in one embodiment, theseswitches are provided as MOSFET transistors.

One goal of the present invention is to control discharge currentthrough the spark plug 50 in such a manner so as to minimize electrodeerosion, thereby maximizing plug life, while maximizing the ability toignite fuel at lean air fuel mixtures, thereby optimizing fuelcombustion. Referring back to FIG. 4, minimization of electrode erosionis defined for the purposes of spark plug 50 as minimizing erosion, dueto current conduction between electrodes 66 and 68, of electrodesurfaces 66a, 66b and 68a. These surfaces define the dimensions of sparkgap 65 and any erosion thereof results in alteration of thesedimensions, which correspondingly affects engine performance and sparkplug life. Controlled energy ignition system 100 is accordinglyoperable, in accordance with one aspect of the present invention, tominimize the spark plug discharge current for arcs 36a and 36b whilealso maintaining sufficient discharge current to permit consistentpropelling of the arc upwardly toward the position indicated by arc 36c.Once the arc is positioned between surface 66c of electrode 66 andsurface 68b of electrode 68, controlled energy ignition system 100 isoperable, in accordance with another aspect of the present invention, toincrease the spark plug discharge current to a level which permitsoptimum ignitability of the air-fuel mixture. Since surfaces 66c and 68bof electrodes 66 and 68 do not directly define any of the boundaries ofspark gap 65, some erosion of surfaces 66c and 68b due to the increasein discharge current is tolerable and will generally not result indegraded engine performance or decreased plug life. The controlledenergy ignition system 100 provides for such discharge current control,and details thereof will now be described with respect to FIGS. 7 and 8.

Referring specifically to FIG. 8, plot 150 represents a dischargecurrent waveform resulting from a known inductive discharge ignitionsystem as described hereinabove. It has been determined throughexperimentation that the peak discharge current between the spark plugelectrodes, resulting in ionization of the spark gap 65 at a breakdownvoltage of V_(BD), generally does not cause significant electrodeerosion if the duration thereof is short (e.g. on the order of fractionsof nanoseconds). In other words, damage to electrode surfaces 66a and68b is minimized if the duration of peak discharge current is short. Ithas further been determined through experimentation that the dischargecurrent must subsequently be controlled to be below a first dischargecurrent threshold I1 within some time period T1 after starting theignition event in order to minimize discharge current-induced electrodeerosion. The discharge current level must, however, be above a minimumcurrent threshold I2 (which is less than I1) at time T1 in order toprovide for subsequent propelling of the arc, under the influence of themagnetic field, in a consistent manner. In one embodiment of spark plug50, I1=150 mA, I2=100 mA and T1 =1 ms, although the present inventioncontemplates other values depending upon the type and configuration ofspark plug and corresponding spark gap.

In accordance with the present invention, system 100 is operable tocontrol the decay of the discharge current after gap ionization tothereby achieve the desired current level of between I1 and I2 at timeT1 as illustrated in the discharge current waveform 152 of FIG. 8. Inone embodiment, control computer 112 is operable to provide such controlby adjusting the value of the variable resistor 118 to thereby controlthe discharge current decay rate. As described hereinabove with respectto FIG. 5, the current density of the discharge current increases withincreasing cylinder pressure, wherein cylinder pressure increases withengine load. Thus, as engine load varies, it is desirable to accordinglycontrol the discharge current level to maintain the discharge currentdensity below a level which results in excessive electrode surfacetemperatures while maintaining the discharge current density above alevel which permits consistent propelling of the arc. Thus, controlcomputer 112 is operable to control the discharge current level aftergap ionization based on current engine load conditions to therebyminimize electrode erosion rate while providing for consistentpropelling of the arc over all engine load conditions. In the embodimentshown in FIG. 8, control computer 112 is preferably operable to providesuch control by first determining engine load, preferably by determiningcylinder pressure based on V_(BD) at gap ionization, mapping cylinderpressure to a desired value of variable resistor 118, and adjusting thevalue of variable resistor 118 to the desired value via output OUT1.Those skilled in the art will, however, recognize that other techniquesmay be used for relating engine load to discharge current level, andthat such techniques may be used to adjust the value of the dischargecurrent to some desired value or range of values within some time periodafter starting the ignition without deviating from the scope of thepresent invention.

Referring now to FIG. 9, one embodiment of a flowchart 160 forcontrolling discharge current level for a time period following gapionization, in accordance with one of the techniques described above, isshown. Algorithm 160 is preferably executable by control computer 112many times per second as is known in the art. Algorithm 160 begins atstep 162 and at step 164, control computer 112 is operable to determinethe breakdown voltage, V_(BD), at gap ionization. Preferably, controlcomputer 112 is operable to execute step 164 by processing the sparkvoltage waveform provided to input IN1 thereof by sensor 110, anddetermining V_(BD) therefrom in accordance with known techniques.Thereafter at step 166, control computer 112 is operable to determinecylinder pressure based on V_(BD). As is known in the art, cylinderpressure is proportional to engine load and cylinder pressure is relatedto V_(BD) via Paschen's law:

    V.sub.BD K.sub.1 *(gap)*(pressure)/ln(K.sub.2 *gap*pressure)(1),

wherein K₁ and K₂ are constants, gap is the width of the spark gap 76(FIG. 3) and pressure is the cylinder pressure. Computer 112 ispreferably operable at step 166 to compute cylinder pressure based onequation (1).

Thereafter at step 168, control computer 112 is operable to determine adesired resistor value based on the cylinder pressure value determinedin step 166. FIG. 10 illustrates one preferred technique for relatingcylinder pressure to desired resistance value, wherein resistance 174 isplotted against cylinder pressure, and wherein engine load indicatorsare shown which correspond to associated cylinder pressure values. Thus,at no load, or idle, conditions, the desired resistor value is high, andthe desired resistor value decreases, preferably according to a chosenfunction, as engine load increases. In an alternative embodiment, theresistance vs. cylinder pressure curve 175 may be used to relatecylinder pressure to a desired resistance value. It is to be understood,however, that the resistance vs. cylinder pressure relationship may takeon any desired shape. In any event, the relationship between desiredresistor values and cylinder pressure values is preferably stored withinmemory portion 146 of control computer 112, and may be representedtherein as an equation (either continuous or piecewise continuous), agraph or plot as shown in FIG. 10, or as a look-up table. Controlcomputer 112 is, in any case, operable at step 168 to map a currentcylinder pressure value to a desired resistor value. Thereafter at step170, control computer 112 is operable to adjust the value of thevariable resistor 118 to the desired resistor value, using any one ormore known techniques, some of which were described hereinabove.Algorithm execution continues from step 170 at step 172 where algorithmexecution is returned to its calling routine, or alternatively loopsback to step 164 for continuous execution of algorithm 160.

It should now be apparent that system 100 is, in accordance with oneaspect thereof, operable to draw current away from spark plug 50following gap ionization to thereby control the discharge current towithin a desired range of discharge current values based on engine loadconditions, gap structure and gap width.

Referring again to FIGS. 7 and 8, system 100 is further operable tocontrollably increase the discharge current to a current level suitablefor igniting the air fuel mixture after the arc has reached the positionillustrated by arc 36c of FIG. 4. As described hereinabove, some erosionof surfaces 66c and 68b is permissible since these surfaces do not formany of the boundaries of spark gap 65. Thus, as the time of air-fuelmixture ignition approaches, control computer 112 is preferably operableto increase the discharge current to a current level at which optimaligniting of the air-fuel mixture occurs. In one preferred embodiment,system 100 is operable to controllably increase the discharge current bysequentially controlling the positions of the various switches 124, 130,136 and 140.

At the beginning of the ignition event, the control signal is applied tothe primary coil 102 which induces a corresponding voltage in thesecondary coil 104 and current through coil 104 increases rapidly, as isknown in the art, until gap ionization occurs, after which the dischargecurrent is controllably decreased as described above. As the gapionization event occurs, switches 124, 130, 136 and 140 are allpreferably open, thereby charging each of the capacitors C1-C4. Controlcomputer 112 is operable to control each of the switches 124, 130, 136and 140 at predetermined time intervals after the ignition event begins,wherein activation of the control signal marks the beginning of eachignition event, and control computer 112 is responsive to the controlsignal supplied thereto via input IN2 to establish a corresponding timemark. In one embodiment of spark plug 50 and corresponding internalcombustion engine (not shown), it has been determined that the dischargecurrent arc reaches the position indicated at 36c of FIG. 4approximately 2.0 milli-seconds after the ignition event begins, and theactual air-fuel ignition event occurs between 3.0-4.0 milli-secondsafter the ignition event begins. Control computer 112 is accordinglyoperable to controllably increase the discharge current level, viacontrol of switches 124, 130, 136 and 140, such that the dischargecurrent is set to a level at which optimal igniting of the air-fuelmixture occurs between 3.0-4.0 milli-seconds. With fast revving engines(i.e., RPM>600), however, these times and durations should becorrespondingly reduced (up to 10 times or more) to prevent possiblevariations in the start of combustion.

In the embodiment illustrated in FIG. 7, control computer 112 ispreferably operable to sequentially close switches 124, 130, 136 and 140to thereby cause the voltage stored in each of the capacitors to beimpressed across corresponding portions of the windings of the secondarycoil 104, thereby sequentially adding supplemental currents (representedby lines 154a, 154b, 154c and 154d in FIG. 8) to the discharge current.Thus, as illustrated in FIG. 8, control computer 112 is operable toclose switch 124 just prior to 1.0 ms after the start of the ignitionevent, close switch 130 just after 1.0 ms after the start of theignition event, close switch 136 just prior to 2.0 ms after the start ofthe ignition event, and close switch 140 just after 2.0 ms after thestart of the ignition event. The resulting effect is to ramp thedischarge current 152 to approximately 170 mA between 3.0-4.0 ms afterthe start of the ignition event, which corresponds to the actual time ofigniting the air-fuel mixture. It is to be understood that the foregoingdescription is illustrative of only one particular application of thedischarge current increasing technique of the present invention, andthat the present invention contemplates providing for the desiredignition discharge current at any time interval following commencementof the ignition event, and by using any number of capacitor/switchcombinations. Those skilled in the art will recognize that the number ofcapacitor/switch combinations used will be dictated by the desired shapeof the discharge current waveform 152 leading up to air-fuel mixtureignition.

Referring now to FIG. 11, an alternate embodiment of a controlled energyignition system 200, in accordance with the present invention, is shown.System 200 is identical in many respects to system 100 of FIG. 7, andlike numbers are accordingly used to identify like elements. Unlikeelements of system 200 include an ignition coil having a primary coil202 inductively coupled to a secondary coil 204 as is known in the art.One end of the primary coil 202 is connected to a capacitor C, to oneend of a voltage source V and to one end of the secondary coil 204, andreceives a control signal for activating system 200. The opposite end ofthe capacitor C is connected to one end of a switch 206 and to one endof a resistor R. The opposite end of the resistor R is connected to anopposite end of the voltage source V, and the opposite end of the switch206 is connected to the anode of a diode D1, the cathode of which isconnected to an opposite end of the primary coil 202 and to one end of asecond switch 210. A control input to switch 206 is connected to anoutput OUT2 of control computer 112 via signal path 208. The oppositeend of switch 210 is connected to ground potential and to one end ofspark plug 50 and variable resistor 118. A control input to switch 210is connected to an output OUT3 of control computer 112 via signal path212. End 204a of secondary coil 204 is connected to voltage sensor 110and to a cathode of a second diode D2, the anode of which is connectedto opposite ends of spark plug 50 and variable resistor 118. Theremaining structure illustrated in FIG. 11 is identical to like numberedcomponents described with respect to FIG. 7.

In operation, control computer 112 is responsive to the control signalprovided at input IN1 thereof to close switch 210 which completes thecoil circuit and causes the spark plug discharge current 220, asillustrated in FIG. 12, to rise. System 200 is preferably operable tocontrol the decrease of the discharge current after gap ionization asdescribed hereinabove, so that the discharge current level is between I1and I2 at a time T1 after starting the ignition event. Thereafter, thedischarge current 220 continues to decay until sometime between 1.0-2.0ms after starting the ignition event, when the control computer 112 isoperable to close switch 206, thereby causing the voltage on capacitorC, which was pre-charged to a suitable voltage by voltage source V, tobe impressed upon the primary coil 202. This induces an additional orsupplemental current in the secondary coil 204, resulting in anapproximately sinusoidal increase in the discharge current 220 asindicated at 222 in FIG. 12. System 200 is thus operable to increase thedischarge current to a suitable level for igniting the air-fuel mixtureat a desired time range after starting the ignition event. Unlike system100, however, system 200 is operable to provide this capability bycontrollably impressing additional voltage on the primary coil 202rather than on the secondary coil 104 as in system 100. Both systemsproduce the expected results, although system 200 is less complicated inthat it does not require high voltage capacitors (which would typicallybe required for capacitors C1-C4 of system 100), and does not requireconfiguring the secondary coil 204 for multiple tap locations. It is tobe understood that the foregoing description is illustrative of onlyanother particular application of the discharge current increasingtechnique of the present invention, and that the present inventioncontemplates providing for the desired ignition discharge current at anytime interval following commencement of the ignition event, and by usingany number of capacitor/switch combinations. Those skilled in the artwill recognize that the number of capacitor/switch combinations usedwill be dictated by the desired shape of the discharge current waveform220 leading up to air-fuel mixture ignition.

While the present invention has been described as being directed totechniques for controlling the discharge current in a diverging gapspark plug having means for magnetically propelling the arc along thediverging gap, those skilled in the art will recognize that the conceptsdescribed herein are applicable to controlling the shape of thedischarge current in ignition systems having conventional spark plugs aswell, and that control of such systems is intended to fall within thescope of the present invention.

Referring now to FIG. 13, one embodiment of a system 300 for controllingignition energy, in accordance with an another aspect of the presentinvention, is shown. One key goal in developing an alternatively fueledengine is to provide for performance commensurate with a conventionalengine, such as a diesel engine, regardless of the type of fuel used.Often times, achievement of this goal requires increasing the engine'scylinder pressure which correspondingly increases demands on theengine's ignition system. For example, increased cylinder pressurerequires increased energy for igniting a spark across the gap of anignition plug which, in turn, increases the rate of ignition plugelectrode erosion and corresponding degradation in engine performance.It has been found, however, that when operating at high cylinderpressures, once the spark is established, i.e. once current begins toflow in an ionized spark gap, the remaining energy stored in theignition coil contributes very little to the combustion event butcontributes greatly to electrode erosion. It is accordingly a goal ofsystem 300 to reduce post-spark energy stored in the ignition system.System 300 is preferably used to minimize or at least decrease electrodeerosion in a conventional spark plug, but may alternatively be adaptedfor use in minimizing or at least decreasing electrode erosion in anarc-propelling plug such as that described hereinabove.

In any case, system 300 includes an ignition coil comprising a primarycoil 102 coupled to a secondary coil 104, as is known in the art,wherein the ignition coil forms part of an ignition system for aninternal combustion engine. System 300 also includes an ignition plug302 having a first electrode connected to one end 104a of secondary coil104 and a second electrode connected to an opposite end 104b ofsecondary coil 104 referenced at ground potential, wherein the ignitionplug electrodes define a spark gap 304 therebetween. As describedhereinabove, ignition plug 304 is, in one embodiment, a conventionalignition plug, but may alternatively be an arc-propelling ignition plugof known construction.

One end of the primary coil 102 is connected to a voltage source V andto one end of a switch 310, and the opposite end of voltage source V isconnected to one end of a switch 306. An opposite end of switch 306 isconnected to the opposite end of the primary coil 102 and to theopposite end of switch 310, and a control input of switch 306 isconnected to a signal path 308 leading from an output OUT1 of controlcomputer 112. Control computer 112 is preferably identical to thatdescribed with respect to FIGS. 7 and 11. A control input of switch 310is connected to a signal path 312 leading from an output OUT2 of controlcomputer 112. Switches 306 and 310 are, in one embodiment, field effecttransistors (MOSFETs, JFETs or the like) having their gates connected tosignal paths 308 and 312 respectively, or bipolar transistors havingtheir bases connected to signal paths 308 and 312 respectively, but itis to be understood that switches 306 an 310 may alternatively be anyelectronically controllable device, such as a relay or the like, that isoperable to controllably conduct current therethrough. In any event,control computer 112 is operable, in accordance with the presentinvention, to control switches 306 and 310 to thereby control currentflow across the gap 304 of ignition plug 302 in such a manner as toeffectuate proper combustion while minimizing ignition plug electrodeerosion, as will be more fully described hereinafter.

System 300 may further include a spark voltage sensor 110 and variableresistor 118, as shown in phantom in FIG. 13, wherein sensor 110 andresistor 118 are identical in structure and operation to like componentsdescribed with respect to FIGS. 7 and 11. If included in system 300,spark voltage sensor 110 has an input connected to end 104a of secondarycoil 104 and an output connected to input IN of control computer 112 viasignal path 114, and variable resistor 118 is connected across spark gap304 and has a control input connected to output OUT3 of control computer112 via signal path 120. Components 110 and 118 may optionally be usedin system 300 to decrease the current level through ignition plug 302below a threshold current level, after a spark is established acrossspark gap 304, in a manner identical to that described hereinabove withrespect to FIGS. 712 to thereby further control the erosion rate of theignition plug electrodes. Those skilled in the art will recognize thatsystem 300 is not limited to an inductive type ignition system, and thatthe concepts of the present invention are applicable to other ignitionsystem types such as, for example, capacitive systems.

Referring now to FIGS. 14-16, the operation of system 300 of FIG. 13will now be described in detail. Under conventional operatingconditions; i.e. without switch 310, sensor 110 or resistor 118, switch306 for a so-called dwell time to thereby charge the primary coil 102.FIG. 14 illustrates a typical primary current waveform 350 during thedwell time occurring between T₀ and T₁. At T₁, switch 306 is opened andthe flux response of the ignition coil couples the energy in the primarycoil 102 to the secondary coil 104 as is known in the art. The secondarycoil 104 is preferably configured to step up the voltage across theprimary coil 102 by some desired ratio, an example of which may be 100:1(secondary voltage to primary voltage ratio). As the energy in theprimary coil 102 is coupled to the secondary coil 104 at T₁, thepotential across the spark gap 302 increases until the air-fuel mixturebetween the gap 302 ionizes. This ionization potential is typicallyreferred to as the spark gap breakdown voltage V_(BD) and is illustratedin FIG. 15. The ionization of the spark gap 302 is followed directly bycurrent flow through the now ionized channel established between theelectrodes of ignition plug 302, as illustrated in FIG. 16, whereby thiscurrent flow beginning just after T₁ begins an in-cylinder combustionevent.

Transfer of energy between the primary coil 102 and secondary coil 104following the dwell time (T₀ -T₁) begins at T₁ and lasts for some timeperiod shown arbitrarily in FIGS. 15 and 16 as T₁ -T4. Thus, underconventional operating conditions, the primary coil 102 is operable toinduce a secondary voltage 352 which begins at T₁, ramps initially to apeak value V_(BD) and thereafter decays until energy transfer iscomplete at T₄. The spark voltage 352 results in a current flow throughsecondary coil 104 as illustrated by secondary current waveform 358 ofFIG. 16. The secondary current waveform begins slightly after T₁ (afterionization of the ignition plug gap 304 as described hereinabove), rampsinitially to a peak value I_(p) and thereafter decays until energytransfer between primary coil 102 and secondary coil 204 is complete atT₄.

The present invention recognizes that once current flow is establishedacross spark gap 304 of ignition plug 302 in a high pressure environment(just after T₁), most of the remaining energy transfer between primarycoil 102 and secondary coil 104 (up to T₄) contributes very little tothe combustion event, but contributes greatly to erosion of theelectrodes of ignition plug 302. Control computer 112 is thereforeoperable, in accordance with the present invention, to close switch 310,preferably at a time T₂ following ionization of, and subsequent currentflow through, gap 304, to thereby cause the remaining energy stored inthe primary coil 102 to be absorbed by primary coil 102 and/ordischarged through switch 310. A resulting spark voltage waveform 354 isillustrated in FIG. 15, wherein control computer 112 is operable toclose switch 306 at time T₂, which results in a faster decay rate of thevoltage across secondary coil 104 and complete dissipation of theignition coil energy by T₃. The net effect is that the spark voltagewaveform 354 is shortened in duration over the conventional sparkvoltage waveform 352 while retaining essentially the same shape. Thecorresponding secondary current waveform 360 (discharge current acrossgap 304 of ignition plug 302) likewise begins a quicker decay at time T₂and decays substantially completely by time T₃ as illustrated in FIG.16. By shortening the duration of secondary current flow (dischargecurrent through plug 302) from T₁ +(slightly after T₁) through T₄ to T₁±T₃, the erosion rate of the electrodes of ignition plug 302 may beminimized, or at least greatly reduced, while sacrificing very little interms of combustion quality.

Control computer 112 may, in accordance with the present invention, beconfigured to determine the appropriate time T₂ to close switch 310according to any one or more techniques. For example, control computer112 may be operable to close switch 310 at some predefined time periodfollowing the controlled opening of switch 306, wherein thepredetermined time period is preferably based on estimated or determinedcylinder pressure conditions. In other words, a time delay DT may bestored in memory 146 as some predetermined delay period, and controlcomputer 112 may be programmed to accordingly close switch 310 at T₁+DT. Alternatively, system 300 may include spark voltage sensor 110wherein control computer 112 is operable to determine dynamic cylinderpressure from equation (1) above. Control computer 112 may, in such anembodiment, have a map, look-up table, one or more equations, and/orgraphical representation relating cylinder pressure values toappropriate T₂ values (or appropriate DT values). In operation, controlcomputer 112 is accordingly operable to monitor the spark voltagewaveform provided thereto on signal path 114, determine therefrom acurrent value of cylinder pressure, and determine from the currentcylinder pressure value an appropriate time T₂ at which to close switch310. Those skilled in the art will recognize other techniques fordetermining an appropriate time T₂ at which to close switch 310, whereinsuch techniques preferably relate T₂ to cylinder pressure conditions,and will further recognize that such other techniques fall within thescope of the present invention.

In accordance with another aspect of the present invention, system 300may further include sensor 110 and variable resistor 118, wherebycontrol computer 112 is operable, in addition to controlling switch 310as just described, to control the ignition plug discharge current tobelow some predefined level within some time period following gapionization or initial current flow across gap 304. In accordance withone or more techniques described hereinabove with respect to FIGS. 7-12,control computer 12 is operable in this embodiment to determine anappropriate value of variable resistor 118, and to control resistor 118to this appropriate value, to reduce the discharge current (secondarycurrent) to below a predefined current limit I_(L) within somepredefined time period T_(x) after energy transfer between the primarycoil 102 and secondary coil 104 begins as shown by the secondary voltagewaveform 356 of FIG. 15 and the secondary current waveform 362 of FIG.16. In one embodiment, the time period T_(x) is measured from T₁.Alternatively, the time period T_(x) is measured from the time at whichgap ionization occurs (detection of V_(BD)) as described hereinabove. Ineither case, the rate of erosion of the electrodes of ignition plug 304may be further reduced, and possibly minimized, by the combined controlof the duration of discharge (secondary) current flow and the magnitudeof the discharge (secondary) current within some predefined time periodafter the transfer of energy from the primary coil 102 to the secondarycoil 104 begins.

While the invention has been illustrated and described in detail in theforegoing drawings and description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiments thereof have been shown and described andthat all changes and modifications that come within the spirit of theinvention are desired to be protected.

What is claimed is:
 1. A system for controlling ignition energy of aninternal combustion engine, comprising:an ignition coil having a primarycoil coupled to a secondary coil, said primary coil responsive to acontrol voltage to induce a spark voltage across said secondary coil;means responsive to a shunting signal for electrically shorting saidprimary coil; and a control computer producing said shunting signalafter said spark voltage is induced across said secondary coil, saidprimary coil thereafter absorbing said spark voltage and accordinglyreducing a duration of said spark voltage induced across said secondarycoil.
 2. The system of claim 1 further including means for sensing saidspark voltage and producing a spark voltage signal correspondingthereto;and wherein said control computer is responsive to said sparkvoltage signal to control production of said shunting signal.
 3. Thesystem of claim 2 wherein said spark voltage signal defines a breakdownvoltage;and wherein said control computer is operable to produce saidshunting signal within a first time period after said voltage signalexhibits said breakdown voltage.
 4. The system of claim 3 furtherincluding an ignition plug having first and second electrodes connectedacross said secondary coil and defining a spark gap therebetween;whereinsaid breakdown voltage of said spark voltage is operable to ionize saidspark gap and produce a discharge current between said first and secondelectrodes.
 5. The system of claim 4 further including a resistorconnected across said spark gap and sized to limit said dischargecurrent below a first threshold current within a second time periodafter said voltage signal exhibits said breakdown voltage.
 6. The systemof claim 5 wherein said resistor is a variable resistor responsive to aresistor control signal to vary a value of said resistor;and whereinsaid control computer is responsive to said spark voltage signal toproduce said resistor control signal as a function thereof to therebycontrol said variable resistor to a suitable resistance value forlimiting said discharge current below said first threshold currentwithin said second time period after said voltage signal exhibits saidbreakdown voltage.
 7. The system of claim 6 further including:a supplyvoltage; and means responsive to an ignition control signal forconnecting said supply voltage across said primary coil to therebyproduce said control voltage there across.
 8. The system of claim 7wherein said control computer is operable to produce said ignitioncontrol signal.
 9. The system of claim 1 further including an ignitionplug having first and second electrodes connected across said secondarycoil and defining a spark gap therebetween;wherein said spark voltage isoperable to ionize said spark gap and produce a discharge currentbetween said first and second electrodes.
 10. The system of claim 9further including a resistor connected across said spark gap and sizedto limit said discharge current below a first threshold current within afirst time period after said spark voltage ionizes said spark gap. 11.The system of claim 10 wherein said resistor is a variable resistorresponsive to a resistor control signal to vary a value of saidresistor;and wherein said control computer is operable to produce saidresistor control signal as a function of said spark voltage to therebycontrol said variable resistor to a suitable resistance value forlimiting said discharge current below said first threshold currentwithin said first time period after said spark voltage ionizes saidspark gap.
 12. The system of claim 11 further including means responsiveto said spark voltage for producing a spark voltage signal correspondingthereto;and wherein said control computer is responsive to said sparkvoltage signal to produce said shunting signal and said resistor controlsignal as functions thereof.
 13. The system of claim 11 furtherincluding:a supply voltage; and means responsive to an ignition controlsignal for connecting said supply voltage across said primary coil tothereby produce said control voltage there across.
 14. The system ofclaim 13 wherein said control computer is operable to produce saidignition control signal.